Carbon molecular sieves are amorphous materials having high surface area and pore sizes near the critical dimensions of small molecules. Early sieves were prepared by the pyrolysis of natural and synthetic precursors such as coconut shells and pitch. Carbon molecular sieves are typically made with pore sizes ranging from 3 to 12 .ANG., pore volumes from 0.2 to 0.5 cc/g, and surface areas from 300 to 1500 m.sup.2 /g. Properties of the sieves can be controlled by choice of carbon precursor, the pyrolysis conditions, and choice of additives, the latter of which usually influences the pore structure. The adsorptive properties of carbon molecular sieves will therefore vary widely depending on the above-mentioned factors.
The early applications of carbon molecular sieves were in the area of adsorption and separation of gas mixtures. The separation of gas molecules, which sometimes differ by as little as 0.2 .ANG. in their critical dimensions, have been achieved with carbon molecular sieves. Such separations are possible due to kinetic effects caused by the dependence of the intermolecular force field on the molecules size. For example, in a mixture of nitrogen and oxygen, the oxygen molecules will experience an attractive force field while the nitrogen molecules will experience a repulsive force field. Commercial gas separations take advantage of such kinetic effects by using a pressure swing operating procedure to separate oxygen and nitrogen.
Further selectivity has been achieved with metal-containing carbon molecular sieves that also provide certain catalytic properties. For example, the preparation, adsorptive, and catalytic properties of a variety of metal-containing carbon molecular sieves have been reported in The Proceedings of the Second Conference on Industrial Carbon Graphite, Society of the Chemical Industry, London, 1966, The Journal Chemical Society D, 8, 477, 1970, and The Conference on Industrial Carbon Graphite, Third Meeting Date, Society Chemical Industry, London, 1970.
The preparation of a class of carbon molecular sieves which are functionalized with certain inorganic oxides and supported metals were reported in the American Chemical Society Symposium Series, 368, 335, 1988. These carbon molecular sieves were designed to combine the molecular sieving properties of the carbon with the surface chemical and physical properties of the particular inorganic oxides. In particular the preparation and characterization of carbon molecular sieves modified with titania, zirconia and alumina additives was described and molecular sieving effects were reported with relationship to pore size distribution, and inorganic oxide type and content. A carbon molecular sieve modified by the addition of titania prepared according to the procedure described in the last mentioned reference was made for purposes of comparison and designated as prior art sorbent composition CMS1.
A commercially available substance marketed as BASF Catalyst R 3-11, will, in its reduced form, remove oxygen from gases. However, the R3-11 sorbent also removes other gaseous species including gaseous sulfur, chlorine and similar materials by an irreversible reaction which permanently lowers the physical and chemisorptive properties of the sorbent. Moreover, temperatures higher than about 230.degree. C. reportedly will permanently damage the R3-11 sorbent.
None of the above-mentioned sorptive materials, provide sorbents which selectively remove oxygen from a gaseous mixture without also simultaneously sorbing substantial amounts of other gaseous species, especially other species having about the same characteristic diameter as oxygen, and without also sorbing gaseous species that permanently decrease the oxygen uptake of the sorbent.
What is therefore needed is a sorbent which will selectively sorb oxygen without sorbing other gaseous species, including gaseous species having about the same characteristic diameter, and without sorbing gaseous species that will permanently damage the sorbent such as sulfur and chlorine.